Named for its boxy shape, this jellyfish has a sting that can cause cardiac arrest and nerve damage if given a hefty enough dose of it.

Image Source: Danza/Shutterstock.com

Its tentacles stretch more than two meters (6.4 feet), and they can lurk in muddy, unclear waters during the warmer months when people are looking to go swimming. It's for these reasons that National Geographic gave this feared aquatic species a 10/10 danger rating!